1. Field of the Invention
The present invention relates to an optical semiconductor resonator, an optical semiconductor device, and an optical module.
2. Description of the Related Art
Some optical semiconductor devices for use in optical communication integrate a resonator portion and a modulator portion among others in one device. This type of optical semiconductor device is manufactured by, for example, a method known as butt-joint (hereinafter abbreviated as BJ) growth in which a multilayer structure such as a multiple-quantum well structure of the modulator portion is formed by crystal growth throughout a wafer, an area that ultimately becomes the modulator portion is masked, other areas than the masked area are removed by etching, and then a multilayer structure of the resonator portion is formed by crystal growth throughout the wafer.
In the case where an optical semiconductor device is manufactured through a BJ growth step, the selective area growth effect (hereinafter referred to as SAG effect) of the modulator area mask, for example, influences the characteristics of the optical semiconductor device. For instance, as described in Japanese Patent Application Laid-open No. 2013-51319, it is a known fact that the photoluminescence (hereinafter abbreviated as PL) wavelength of an optical waveguide varies depending on its location with respect to an end of the mask.
The resonator portion (laser portion) integrated in an optical semiconductor device needs to secure stability in emission wavelength in order to enhance optical communication quality. It is therefore necessary to keep the side-mode suppression ratio (SMSR) at a certain level or higher so that the oscillation mode does not change in the case of a change in drive current applied to the laser portion or in the case of deterioration with time of the laser portion.
However, when the laser portion is formed by BJ growth in the manner described above, the SAG effect of the BJ mask causes the laser multilayer to increase in film thickness toward the end of the BJ mask from a rear end surface of the laser portion, resulting in non-uniform composition wavelength in the optical axis direction. Consequently, an effective refraction index n of the laser is non-uniform in the optical axis direction, which makes a DFB wavelength λDFB expressed as 2 nΛ (Λ represents the diffraction grating pitch) non-uniform in the optical axis direction. With the effective refraction index being non-uniform in the optical axis direction, in a case of distributed feedback (DFB) lasers, for example, the frequency symmetry of the threshold gain is lost in each longitudinal mode and the threshold gain decreases on the side where the frequency is low. This reduces a threshold gain difference between a zero-order oscillation mode where the threshold gain is minimum and a longitudinal mode (sub-mode) where the threshold gain is second smallest. In other words, this reduces an intensity difference between the zero-order oscillation mode and the sub-mode, which may lower the SMSR yield.